Typically, during semiconductor processing, a plasma etch process is used to remove or etch material along fine lines or within vias or contacts patterned on a semiconductor substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a processing chamber and etching exposed areas of the substrate through the pattern.
Once the substrate is positioned within the chamber, it is etched by introducing an ionizable, dissociative gas mixture into the chamber at a pre-specified flow rate, while adjusting a vacuum pump to achieve a processing pressure. Then, plasma is formed when a portion of the gas species is ionized by collisions with energetic electrons. The gas may be ionized by direct current, radio frequency, microwave energy, or other energy sources known to the art. The energetic electrons dissociate some of the gas species in the gas mixture to create reactant species suitable for the exposed-surface etch chemistry. Once the plasma is formed, exposed surfaces of the substrate are etched by the chemistry at a rate that varies as a function of plasma density, average electron energy, and other factors. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to more selectively act upon various desired features (e.g., trenches, vias, contacts, etc.) in the exposed regions of a substrate. The exposed regions of the substrate where etching is required are typically formed of materials such as silicon dioxide (SiO2), poly-silicon and silicon nitride, for example.
While plasma etching has proven to be generally effective, process efficiency may be negatively impacted by a variety of factors. For example, undesirably high average electron energies tend to impede ion formation in the presence of process chemicals, and thus result in reduced dissociative attachment at the substrate. Attempts to attenuate the negative effects noted above have included the introduction of Multi Pole Magnet (MPM) assemblies and Dipole Ring Magnetron (DRM) assemblies designed to advantageously select low energy electrons near a substrate.
However, these attempts to select low electron energies by exposing plasma to magnetic fields have produced undesirable side effects. For example, in plasma processing chambers utilizing a microwave plasma (energy) source, previous magnetic assemblies yielded magnetic field profiles that bulge outwardly toward the plasma source and toward the substrate proximate the center axis of the processing chamber. This produces a strong magnetic field in the over-dense surface wave plasma (“SWP”) region near the plasma source, as well as a strong magnetic field in the region of the substrate. The magnetic field lines produce complex wave propagation patterns in the SWP region, leading to plasma non-uniformity in the substrate region. Each of these side effects adversely impacts the uniformity of the semiconductor devices being processed. Apparatus and methods to reduce the bulging field in those critical regions are costly and complicated.
Therefore, an apparatus and method for uniformly applying low average electron energy plasma etch, while providing a highly controlled region of magnetic influence, is needed.